1. Field of the Invention
The present invention generally relates to the processing of biomass, and more particularly to a self-contained method and system for generating solids from biomass.
2. Discussion of the Background
The use of solid fuels is the world's largest energy market. In the United States, solid fuels are used primarily for generating electric power and in metallurgic and cement manufacturing processes. This market is dominated by non-renewable resources, principally coal, and to a lesser extent petroleum coke. Biomass sources, which are generally considered to be renewable, form less than 5% of the U.S. Market.
There is an urgency to switch to energy sources that will have less of an environmental impact, especially with regards to the emission of greenhouse gases. Biomass sources are an attractive alternative to conventional solid fuels, but high transportation costs and low energy density of the biomass materials have hindered their widespread use.
Methods to improve the fuel value and physical properties of biomass range include drying the biomass to remove moisture without chemically altering the biomass, and producing charcoal from the biomass, where the biomass is chemically altered into fixed carbon. Drying is accomplished at temperatures below 120 C, while charcoal production requires temperatures above 500 C. Both drying and producing charcoal are incomplete solutions, and do not enable the access to remote biomass resources.
Drying the biomass combined with grinding and pelletizing the resultant fuel produces a fuel with energy density of between 7,000 Btu per pound (16,000 kJ/kg) and 8,000 Btu per pound (19,000 kJ/kg), and a density of 0.6 g/cm3 (600 kg/m3) and 1 g/cm3 (1,000 kg/m3), and is something of an improvement. However the pellets are intolerant to water, are capable of spontaneous combustion, and are thus difficult to store.
Producing charcoal is inefficient, with only 20% to 30% of the energy in the original biomass preserved in the charcoal. So much energy is lost that producing charcoal for fuel is discouraged except for use in metallurgical processes, where it is mandatory and thus unavoidable. In addition, densifying charcoal requires a binder, a severe limitation when operating remotely. In either case, the resultant fuel is unsatisfactory for widespread application to industrial combustion processes.
Thus there is a need in the art for a method and apparatus that permits for the widespread use of biomass as a solid fuel. Such a method and apparatus should be inexpensive to implement and should produce a fuel that is compatible with apparatus that use conventional solid fuels.